1. Field of the Invention
The present invention relates generally to the structure of a polyimide multilayer wiring substrate having multiple wiring layers, wherein polyimide resin is employed as interlaminar insulation, and a manufacturing method of said substrate and, more particularly to a structure of polyimide resin layers and a method for laminating said layers.
2. Description of the Related Art
The use of a multilayer printed wiring substrate as a wiring substrate for carrying LSI chips is well known. The multilayer printed wiring substrate comprises copper-plated layered sheets as core materials and prepreg as adhesives for the core materials, the core materials and prepreg being alternately laminated integrally with each other by application of heat pressing. After the core materials and prepreg are integrally laminated, through-holes are drilled through the substrate by means of a drill and the inner walls of the through-holes are plated with copper to effect electrical connections between the laminated sheets.
In recent years, a polyimide-ceramic multilayer wiring substrate in which polyimide resin is applied to a ceramic substrate to effect interlaminar insulation has been brought into use as a wiring substrate for use in large computers. This type of substrate requires wiring of greater density than that of the multilayer printed wiring substrate. Said polyimide-ceramic multilayer wiring substrate is produced by a method comprising a series of polyimide resin insulation layer formation steps, in which polyimide precursor varnish is applied to a ceramic substrate and dried to form a coating film in which viaholes are formed, and a wiring layer formation steps, in which photolithography, vacuum vapor deposition and plating methods are utilized, these steps being repeated to form polyimide multiple wiring layers.
Apart from the aforementioned method for forming a polyimide-ceramic multilayer wiring substrate, there is a method for forming a multilayer wiring substrate by forming a wiring pattern on a polyimide sheet, positioning said sheet on a ceramic substrate, and successively performing the aforementioned steps under pressure to form laminated sheets. By this method, a signal layer is formed one sheet at a time, thereby enabling the selection and lamination of sheets free from defects, with the result that the manufacturing yield can be raised.
In the aforesaid multilayer printed wiring substrate, electrical connection between laminated sheets is established through a through-hole formed by means of a drill; however, it is impossible to form a through-hole having a very small diameter, and the number of connections that can be provided by the through-holes is therefore limited. Since one connection between laminated sheets requires one through-hole, as the number of laminations increases, the capacity for accommodating signal wirings decreases, thereby making it difficult to form a multilayer printed wiring substrate of high wiring density.
A polyimide-ceramic multilayer wiring substrate which has recently been developed to remedy the drawback of the conventional multilayer printed wiring substrate requires repeated processes comprising application of precursor varnish to a ceramic substrate, drying said varnish, formation of viaholes, and curing the substrate, these processes being repeated as many times as the number of laminations of polyimide insulation layers, resulting in a very lengthy lamination process of the multilayers. Since the formation process of the polyimide insulation layers is performed repeatedly, the polyimide resins of lower layers of the multiple wiring layers are repeatedly subjected to the heat stress of the curing process, resulting in the degradation of the polyimide resins. Further, the polyimide multiple wiring layers are laminated in succession, layer after layer, thereby making it difficult to improve manufacturing yield.
If the lamination is carried out sheet after sheet, a process which has been developed to improve the manufacturing yield, there are also the drawbacks that since the layers are laminated layer after layer under pressure, as the number of layers increases, heat stress is imparted to the polyimide resin of the lower layers and causes the degradation of the lower layers, and in addition, the manufacturing process is time-consuming.